One stage method for acid metal removal and bleach

ABSTRACT

There is provided a process for chemically pretreating reclaimed cellulose fibres to be used in the production of moulded bodies from regenerated cellulose, wherein the pretreatment includes one stage, in which stage acid metal removal and acid oxidative bleaching are carried out together. Advantages include that the propensity of the regenerated cellulose to clog when flowing in a tube and through a nozzle is reduced. This is believed to be an effect of an efficient metal removal. The need for additional bleaching steps and/or metal removing steps is reduced or even eliminated. A one-stage method is more efficient, faster and less costly compared to a multi-stage method according to the prior art. From an environmental perspective, acidic metal removal is preferred over removal by chelating agents such as EDTA.

TECHNICAL FIELD

The invention relates to a one-stage step in a process for regeneratingreclaimed cellulose, where acid metal removal is combined together withacid oxidative bleach in one single stage.

BACKGROUND

WO 2012/057684 discloses a process for the derivatization of cellulosecomprising the sequential steps: a) mixing cellulose with a viscositybelow 900 ml/g with an aqueous solution to obtain a liquid, whereinparticles comprising cellulose in said liquid have a diameter of maximum200 nm, wherein the temperature of the aqueous solution is below 20° C.,and wherein the pH of the aqueous solution is above 12, b) subjectingthe liquid to at least one of the steps: i) decreasing the pH of theliquid with at least 1 pH unit, ii) increasing the temperature by atleast 20° C., and c) derivatization of the cellulose.

WO 2010/124944 discloses a process for the hydrolysis of cellulosecomprising the sequential steps (a) mixing cellulose with a viscositybelow 900 ml/g with an aqueous solution to obtain a liquid, whereinparticles comprising cellulose in said liquid have a diameter of maximum200 nm, wherein the temperature of the aqueous solution is below 35° C.,and wherein the pH of the aqueous solution is above 12, (b) subjectingthe liquid to at least one of the steps: (i) decreasing the pH of theliquid with at least 1 pH unit and (ii) increasing the temperature by atleast 20° C., and (c) hydrolysing the cellulose. Moreover, there isdisclosed glucose manufactured according to the method and ethanolmanufactured from the glucose.

WO 2013/124265 discloses a method for regeneration of a cellulosecontaining material, comprises the steps: a) exposing the cellulosecontaining material to oxygen with an alkali aqueous solution at a pH ofat least 9, and a temperature of at least 20° C., b) dispersing thecellulose containing material in the alkali aqueous solution, whereinthe temperature of the alkali aqueous solution is lowered below 15° C.,and wherein the pH of the alkali aqueous solution is above 9, c) addingan organic solvent to the dispersion to precipitate cellulose, and d)separating the precipitated cellulose by at least one method selectedfrom filtering and centrifugation. The method makes it possible tomaintain a high alkali pH value in the process, which saves costs sincethe pH value does not have to be lowered by additions of variousadditives.

WO 2018/104330 discloses a cellulose based fibre made of i) a cellulosedissolving pulp, and ii) a recycled cellulose textile, which is treatedto swell the cellulose with a reducing additive and a) bleached withoxygen at alkaline conditions with a pH in the range 9-13.5 and/or b)bleached with ozone at acid conditions below pH 6, wherein the cellulosebased fibre is manufactured with one selected from a Viscose process anda Lyocell process.

WO 2018/073177 discloses method for recycling textiles comprisingcellulose with the following steps of:

optionally disintegrating the textile, swelling the cellulose, underreducing conditions, wherein at least one reducing agent is present atleast during a part of the swelling, and then performing at least one ofthe following two bleaching steps in any order: i) bleaching thematerial with oxygen at alkaline conditions with a pH in the range9-13.5, and ii) bleaching the material with ozone at acid conditionsbelow pH 6.

WO 2015/077807 discloses a process for pretreating reclaimed cellulosefibres to be used in the production of moulded bodies from regeneratedcellulose by the Viscose or Lyocell process, wherein the treatment ofthe reclaimed cellulose fibres includes a chemical metal removing stageand an oxidative bleaching stage. The process is described as amultistage process. The metal removing stage can be acidic washing ortreatment with a complexing agent, or a combination of both. Theoxidative bleaching stage can be treatment with peroxide, oxygen orozone.

WO 2016/123643 discloses a process for producing a man-made cellulosicmoulded body using a reclaimed man-made cellulosic raw material,including the steps of forming a cellulose solution by dissolution ofcellulosic raw material, the extrusion of the cellulose solutionobtained to form a moulded body, and coagulation and regeneration of thecellulose to obtain the man-made cellulosic moulded body, wherein thereclaimed man-made cellulosic raw material is mixed with a virgincellulosic raw material prior to forming the cellulose solution. WO2016/123643 discloses that an acidic washing step and a treatment withan aqueous solution of a complexing agent may be combined in one processstep, by adding the complexing agent to the acidic washing liquor.

Problems in the state of the art include how to improve the reduction ofmetal content of regenerated cellulose. Another problem is how tofurther reduce the clogging properties of regenerated cellulose. Ingeneral, it is also desirable to provide a simpler and more efficientmethod to regenerate cellulose.

SUMMARY

It is an object of the present invention to alleviate at least some ofthe problems in the prior art and to provide a method for pre-treatingreclaimed cellulose fibres.

There is provided a process for chemically pretreating reclaimedcellulose fibres to be used in the production of moulded bodies fromregenerated cellulose, wherein the pretreatment includes one stage, inwhich stage acid metal removal and acid oxidative bleaching are carriedout together.

The propensity of the regenerated cellulose to clog when flowing in atube and through a nozzle is reduced. This is believed to be an effectof an efficient metal removal.

The need for additional bleaching steps and/or additional metal removingsteps is reduced or even eliminated.

A one-stage method is more efficient, faster and less costly compared toa multi-stage method according to the prior art.

The invention makes it possible to remove metals efficiently withoutusing complex formers such as EDTA, which may be an environmental issuedue to the longevity of EDTA and similar compounds.

DETAILED DESCRIPTION

The following detailed description discloses by way of examples detailsand embodiments by which the invention may be practised.

It is to be understood that the terminology employed herein is used forthe purpose of describing particular embodiments only and is notintended to be limiting since the scope of the present invention islimited by the appended claims.

If nothing else is defined, any terms and scientific terminology usedherein are intended to have the meanings commonly understood by those ofskill in the art to which this invention pertains.

There is provided a process for chemically pretreating reclaimedcellulose fibres to be used in the production of moulded bodies fromregenerated cellulose, wherein the pretreatment includes one stage, inwhich stage acid metal removal and acid oxidative bleaching are carriedout together.

As known in the prior art reclaimed cellulose can be regenerated andused for production of moulded bodies. Examples of such processesinclude but are not limited to: the Lyocell process, in particular usingaqueous amine oxide, such as 4-methylmopholine N-oxide (EP 0356419 andEP 0584318), the viscose process (Kurt Götze, Chemiefasern nach demViskoseverfahren, 1967) and the Modal process (AT 287905).

It has been discovered that by using acid metal removal and acidoxidative bleaching together in one single stage, several advantages canbe achieved compared to the prior art. As shown in the appendedexperimental results the acid metal removal is more efficient comparedto removal by complex formation such as addition of EDTA. Further, itcan be seen from the experiments that the clogging is reduced forcellulose treated according to the invention.

Without wishing to be bound by any specific scientific theory theinventors believe that the removal of metal ions reduces the propensityof the cellulose chains to form clusters and cause clogging and otherproblems. It is believed that in particular multivalent ions such asdivalent and trivalent ions, such as Ca²⁺ contribute to the formation ofclusters due to their strong electrostatic interactions. For instance,carboxylated cellulose nanofibrils can form hydrogels in the presence ofdivalent and trivalent ions and form interconnected porous nanofibrilnetworks. When the metal ions are removed, the clogging is reduced asmeasured in the appended experiments.

Compared to a multistep method such as the one described in WO2015/077807, several advantages are achieved. First one single stage isless costly, faster, and more efficient than two or more stages.Secondly, the clogging propensity of the regenerated cellulose isreduced to a greater extent as shown by the appended experimental data.

Again without wishing to be bound by any particular scientific theory,the inventors believe that the combination of acid oxidative bleachingand acid metal removal in one step can oxidize at least some of thechemical structures which bind the metals and thus facilitates theremoval of the metals. Further the inventors believe that an additionaleffect may be that the oxidative bleaching give some degradationproducts, which in some cases may form complexes with the metal ions andthus also facilitate the removal of metal.

Thus the combination of an acid oxidative bleach and an acid metalremoval in one step give a more efficient removal of metal.

Metal ions are present in recycled cellulose and it is desired to reducethe content of such metal ions.

The stage is one single stage and is intended to be used as a stage inthe regeneration of reclaimed cellulose, including recycled clothing,for instance clothing comprising cotton.

It is conceived that the present stage according to the invention is tobe carried out together with additional steps and stages in theregeneration of reclaimed cellulose. Additional stages are known in theart and can easily be combined with the stage according to the inventionby a skilled person. A number of additional stages are suitablyperformed in the regeneration of reclaimed cellulose as described in theprior art. Buttons, zips and other objects of solid metal are in oneembodiment removed before the stage. A mechanical treatment is in oneembodiment, carried out before the present stage, such as for instanceshredding and milling. A chemical treatment step is in one embodimentcarried out before the stage according to the invention. Such a chemicaltreatment step may include a partial or entire dissolution of thereclaimed cellulose.

In one embodiment, a step of removing non-cellulosic fibres isperformed. Examples of such non-cellulosic include but are not limitedto fibres comprising polyester, elastan, acryl. In one embodiment, theremoval of non-cellulosic fibres is carried out by flotation.

Any suitable acid can be used in order to obtain an acidic pH during thestage, i.e. a pH value below 7. In order to obtain appropriate pH,mixtures of acids can be used. In one embodiment, a carboxylic acid ispresent during the stage. The wording that an acid is present during thestage means that it may be present during at least a part of the stage,for instance an acid may be present at the beginning of the stage inorder to create the desired pH value and during the stage one or moreadditional acids can be added. In one embodiment, at least one or moreacids are present during a part of the stage. In an embodiment where anacid is present during a part of the stage, it should be ensured thatthe pH value is acidic as desired during the entire stage, for instanceby presence of acid(s) also in the beginning of the stage. Examples ofacids which may be present as only acids or together with other acidsinclude but are not limited to hydrochloric acid, formic acid, citricacid, acetic acid. Mixtures including one or more of these acids arealso encompassed. Mixture including these acids together with additionalacids are also encompassed. In one embodiment, at least one acidselected from the group consisting of hydrochloric acid, formic acid,citric acid, acetic acid and any mixture thereof, is present during thestage. In one embodiment, a mixture of acids is used. For instance,hydrochloric acid can be mixed with a weaker acid such as acetic acid inorder to achieve the desired pH. The fact that at least one acid ispresent during the stage means that the acid can be added in thebeginning of the stage, or before the stage or a combination thereof. Inone embodiment, the acid is added in the beginning of the stage.Addition of an acid in the beginning of the stage or before the stagedoes not exclude the possibility of addition of further acid during thestage.

In one embodiment, the pH is in the interval 1-3 during the stage. Inanother embodiment the pH is in the interval 2-3 during the stage. In analternative embodiment, the pH is in the interval 0-4 during the stage.In yet another embodiment the pH is in the interval 1-5 during thestage. In a further embodiment, the pH is in the interval 1.5-5 duringthe stage. In a still further embodiment, the pH is in the interval 0-5during the stage. In one embodiment the pH is in the interval 1.5-4.5.

In one embodiment, the temperature during the stage is in the interval40-60° C. In another embodiment, the temperature during the stage is inthe interval 30-75° C. The temperature during the stage can vary. Forinstance, the temperature can be high and in the upper part of theinterval in the beginning of the stage and lower towards the end. Also alower initial temperature followed by a rise in the temperature isconceivable. It is not necessary that the temperature is within theranges during the entire stage, temperatures below and above the rangesare also conceivable. In alternative embodiments, the temperature isoutside the ranges during the entire stage.

In one embodiment, at least one weak acid with a pKa above 3 and below 7is present during the stage. 3<pKa<7. A weak acid has the advantage thatthe cellulose is not hydrolysed by the acid, or at least not hydrolysedto any significant degree.

In one embodiment, acetic acid is present during the stage. Withoutwishing to be bound by any particular scientific theory, the inventorsbelieve that acetic acid is beneficial for removing dye. Although themechanisms behind this are not fully understood, the inventors believethat addition of acetic acid gives an improvement in the removal ofcertain dyes. The stage according to the present invention can suitablybe combined with additional bleaching stages in order to fully removeany remaining dyes. Such additional bleaching stages are known anddescribed in the prior art.

In one embodiment, sulphuric acid is not present during the stage. Acommon impurity in reclaimed cellulose such as reclaimed cotton iscalcium. When sulphuric acid is utilized, Ca²⁺ ions can react with thesulphuric acid and form CaSO₄ (gypsum), which makes the process lessefficient. It is still possible to use sulphuric acid as evidenced bythe examples, but longer treatment time and/or a higher concentration ofsulphuric acid would be necessary. Further less efficient metal removalcan be expected. Thus the use of sulphuric acid is often less preferred.However, in an alternative embodiment sulphuric acid is present duringthe stage. If used, sulphuric acid should be used at higherconcentration and/or with longer treatment time.

During the acidic metal removal stage, an acidic oxidative bleach takesplace simultaneously in the same stage in order to improve theefficiency. In one embodiment, hydrogen peroxide is present during thestage. The dose of hydrogen peroxide is in one embodiment 2 to 40 kghydrogen peroxide per odtp. In one embodiment, ozone is present duringthe stage. In one embodiment, the dose of ozone is 0.1 to 6 kg ozone perodtp. Since the acidic metal removal and the acidic oxidative bleach arecarried out together in the same stage, the pH is acidic for allconceivable oxidative bleaching additives.

The oxidative bleaching agent is added before the step starts or at thestart of the stage. In one embodiment, an oxidative bleaching agent isadded in the beginning of the stage and at least one oxidative bleachingagent is added during the stage. In one embodiment, at least oneoxidative bleaching agent is added in the beginning of the stage and atleast one additional different oxidative bleaching agent is added duringthe stage. In one embodiment, ozone is added in the beginning of thestage and hydrogen peroxide is added during the stage at a later pointin time. In another embodiment hydrogen peroxide is added in thebeginning of the stage and ozone is added during the stage. Also othercombinations of additions of oxidative bleaching agents are encompassed.

In one embodiment, the stage with both acid metal removal and acidoxidative bleaching is the only acidic chemical metal removal step inthe process. Even if the detailed mechanisms are not fully investigatedthe inventors believe that the acidic metal removal and the acidicoxidative bleach should be carried out together in one stage. If acidicmetal removal and the bleach are carried out separately the efficiencyis lower as seen in the examples. Thus the combined acid metal removaland acid oxidative bleaching is in one embodiment the only chemicalmetal removal stage in the process, i.e. no additional acid chemicalmetal removal stages are carried out before or after the stage accordingto the invention. The metal removal according to the invention is soefficient that it is more economical to use only one efficient stage andno additional acidic metal removal stages. In one embodiment, the acidicmetal removal in the stage is the only acidic metal removal.

In one embodiment, the stage is carried out during a time in theinterval from 1 to 120 minutes. In another embodiment the stage iscarried out during at time in the interval 1-60 minutes. In yet anotherembodiment the stage is carried out during 2-60 minutes. In yet anotherembodiment the stage is carried out during 5-60 minutes. In yet anotherembodiment the stage is carried out during 10-60 minutes. Longertreatment times can also be used, but can also be less economical. Thusin one embodiment, the stage is carried out during a time of at least 1minute, at least 2 minutes, at least 5 minutes, or at least 10 minutes.

The reclaimed cellulose fibres are in one embodiment, cotton fibres. Inone embodiment, the cellulose fibres originate from pre-consumercellulose containing waste such as cotton. Pre-consumer cellulosecontaining waste includes but is not limited to combing waste andcuttings. In one embodiment, the cellulose fibres originate frompost-consumer cellulose containing waste such as cotton. Post-consumercellulose containing waste includes but is not limited to laundry wasteand used clothes. In one embodiment, the cellulose fibres comprise pulpprepared from cotton rags. In one embodiment, the reclaimed cellulose ismechanically shredded or milled prior to the use.

In one embodiment, the production of moulded bodies is made with theViscose process. In one embodiment, the production of moulded bodies ismade with the Lyocell process. In one embodiment, the production ofmoulded bodies is made with the Modal process. These processes forproduction of moulded bodies are known in the art and can be performedby a skilled person. The intrinsic viscosity of the regeneratedcellulose is suitably adjusted as needed and as known depending on theintended production process for the moulded bodies. For instance, for aViscose process the intrinsic viscosity can be adjusted to a value inthe range 350-650 ml/g. For instance, for a Lyocell process theintrinsic viscosity can be adjusted to 350-500 ml/g.

EXAMPLES

In order to show the advantageous properties of the present invention aclogging test was performed. A mixture to be tested was allowed to runthrough a narrow passage. The times for the passing of 25-50 ml wasrecorded as well as the time for the passing of 125-150 ml. Thedifference was recorded as delta T (Δt).

Example 1

Reactivity test based on wash, white jersey fabric. The samples weretreated mechanically by cutting in pieces 1×1 cm and thereafter treatedin a mixer for 40 seconds. This shredding served to open the fiberstructure. After the mechanical step, the material was treated in achemical step where the material was bleached at high pH, i.e. a pHabove 7.

The following three samples were prepared:

H₂O (1) EDTA (2) H₂SO₄ (3) Mass fabric(g, od) 50 50 50 EDTA (g/mL) N/A0.3 N/A H₂SO₄ (mol/L) N/A N/A 0.01M (pH 2)

Samples with a weight of 3.75 g were treated with ozone during 10 minwith an amount corresponding to 238 kg ozone/hour. Ozone treated pulpsamples were then wetted overnight and diluted to a concentration of 3.5wt %. The initial ozone treatment was a separate oxidative bleachingstep. Thereafter each pulp sample was washed with the additive accordingto the above table.

-   -   H₂O (1): was washed with 1.5 l deionized water.    -   EDTA (2): was washed with EDTA-solution (2 liters, 60° C.) and        thereafter with 1.5 l deionized water.    -   H₂SO₄(3): was washed with H₂SO₄ (2 liters, 0.01 M, 60° C.)        followed by 1.5 l deionized water.

The second wash was a different metal removal step.

After the different washes the samples were diluted to a concentrationof 9 wt % and the pH was adjusted to 6 for all samples with NaOH and ifneeded with H₂SO₄.

Delta T (Δt) was measured for the samples and the results are shown inthe below table.

Results

t1 (25-50 t2 (125-150 Sample ml) [s] ml) [s] Δt [s] H₂O (1) 55 215 160EDTA (2) 24 53 29 H₂SO₄ (3) 51 177 126

As can be seen the acidic metal removal is not very efficient comparedto the control sample with water. There is still a considerableclogging, which is attributed to metal ions present in the regeneratedcellulose. Metal removal with the chelating agentethylenediaminetetraacetic acid (EDTA) is more efficient compared tosulphuric acid. However, environmental safety has raised concerns aboutthe low biodegradability of aminopolycarboxylates such as EDTA.

Example 2

Reactivity test based on wash, white jersey fabric. First, the materialwas subjected to a mechanical step and thereafter in a chemical step,both as in example 1. Thereafter the following three samples wereprepared.

(1)H₂SO₄ (2)HCl (3)Acetic acid Mass 50 50 50 fabric(g, od) H₂O₂ (kg/ton)2 2 2

The samples were wetted overnight and diluted to a concentration of 3.5wt %. Thereafter the pH value of each pulp sample was adjusted and H₂O₂was added.

-   -   H₂SO₄: pH was adjusted to 2 with H₂SO₄.    -   HCl: pH was adjusted to 2 with HCl.    -   Acetic acid: pH was adjusted to 2.4 with acetic acid.

After the pH adjustment, the samples had a dwell time of about 20minutes before wash with deionized water. This gave a stage withcombined acidic metal removal and acidic oxidative bleach. All sampleswere washed with 1.5 liters deionized water after filtering.

After the washing, the samples were diluted to a concentration of 9 wt %and the pH was adjusted to 6 for all samples with NaOH. If needed the pHwas also adjusted with H₂SO₄ for the H₂SO₄-sample and with HCl for theHCl-sample and the acetic acid-sample.

Delta T (Δt) was measured for the samples and the results are shown inthe below table.

Results

t1 (25-50 ml) t2 (125-150 ml) Sample [s] [s] Δt [s] H₂SO₄ 17.6 26.3 8.7HCl 7.5 12.6 5.1 HAc 8.8 12.5 3.7

As can be seen there is a noticeable improvement in the Δt by using thetreatment according to the invention, in particular when not usingH₂SO₄.

Example 3

Tests based on unbleached denim. Denim samples were treated mechanicallyby cutting in pieces and grinding. This shredding served to open thefiber structure.

After the mechanical step, the material was subjected to a step withcombined acid metal removal and acid oxidative bleach. In this stepacetic acid was charged with 12 kg/odt and hydrogen peroxide was chargedwith 5 kg/odt. The pH was 2 and the temperature was 55° C. The treatmenttime was 20 minutes.

The content of various metals was measured in the denim before thetreatment and after the treatment. The measurement results are shown inthe below table. The results are calculated as mg of metal per kg ofmaterial in dry state.

After combined acid oxidative bleach and Unbleached acid metal MetalUnit Denim removal Fe mg/kg 76.3 32.3 Ca mg/kg 446 19.7 Si mg/kg 366 356Co mg/kg <0.2 <0.2 Cr mg/kg 1.3 0.6 Mg mg/kg 155 19.6 Mn mg/kg 1.7 0.3Mo mg/kg <0.2 <0.2

As can be seen there is a significant reduction of various metals in thedenim, possibly with the exception of the metalloid Si, where there is aslight reduction. The amounts of Co and Mo were so small that they weredifficult to measure with the utilized method.

Example 4

Example 3 was repeated, but with different conditions. Acetic acid wascharged with 18 kg/odt and hydrogen peroxide was charged with 2 kg/odt.The temperature was 35° C. The treatment time was 60 minutes. Theexample was repeated twice. Once with pH 3.5 and once with pH 2.5.

The content of various metals was measured in the denim before thetreatment and after the treatment in the same way as for example 3. Theresults are shown in the below table.

After After combined combined acid acid oxidative oxidative bleach andbleach and acid metal acid metal Unbleached removal removal Metal UnitDenim pH 3.5 pH 2.5 Fe mg/kg 76.3 30.3 17.3 Ca mg/kg 551 182 92.2 Simg/kg 385 197 125 Co mg/kg 0.12 0.045 0.043 Cr mg/kg 1.8 1 0.7 Mg mg/kg83.9 23.3 15.5 Mn mg/kg 1.4 0.5 0.4 Mo mg/kg 0.172 0.0235 0.045

As can be seen the content of all the metals including the metalloid Sidecreases significantly. It can also be seen that the lower pH worksbetter.

1. A process for chemically pretreating reclaimed cellulose fibres to beused in production of moulded bodies from regenerated cellulose, whereinthe pretreatment includes one stage, in which stage acid metal removaland acid oxidative bleaching are carried out together.
 2. The processaccording to claim 1, wherein at least one acid selected from the groupconsisting of hydrochloric acid, formic acid, citric acid, acetic acidor any mixture thereof, is present during the stage.
 3. The processaccording to claim 1, wherein at least one weak acid with a pKa above 3and below 7 is present during the one stage.
 4. The process according toclaim 1, wherein acetic acid is present during the one stage.
 5. Theprocess according to claim 1, wherein sulphuric acid is not presentduring the one stage.
 6. The process according to claim 1, whereinsulphuric acid is present during the one stage.
 7. The process accordingto claim 1, wherein hydrogen peroxide is present during the one stage.8. The process according to claim 1, wherein ozone is present during theone stage.
 9. The process according to claim 1, wherein the one stagewith both acid metal removal and acid oxidative bleaching is the onlyacid chemical metal removal step in the process.
 10. The processaccording to claim 1, wherein the one stage is carried out during a timein the interval from 1 to 60 minutes.
 11. The process according to claim1, wherein the temperature during the one stage is in the interval30-75° C.
 12. The process according to claim 1, wherein a subsequentproduction of moulded bodies from regenerated cellulose is carried outwith one method selected from the group consisting of the Viscoseprocess, the Lyocell process, or the Modal process.
 13. The processaccording to claim 1, wherein the one stage is carried out during a timein the interval from 1 to 120 minutes.
 14. The process according toclaim 1 wherein the pH during the one stage is in the interval 1.5 to4.5.